Re: [PATCH 2/5] xfs: open code inc_inode_iversion when logging an inode

[Eric Sandeen <sandeen@sandeen.net>]

On 10/31/13, 11:27 PM, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
> 
> Michael L Semon reported that generic/069 runtime increased on v5
> superblocks by 100% compared to v4 superblocks. his perf-based
> analysis pointed directly at the timestamp updates being done by the
> write path in this workload. The append writers are doing 4-byte
> writes, so there are lots of timestamp updates occurring.
> 
> The thing is, they aren't being triggered by timestamp changes -
> they are being triggered by the inode change counter needing to be
> updated. That is, every write(2) system call needs to bump the inode
> version count, and it does that through the timestamp update
> mechanism. Hence for v5 filesystems, test generic/069 is running 3
> orders of magnitude more timestmap update transactions on v5
> filesystems due to the fact it does a huge number of *4 byte*
> write(2) calls.
> 
> This isn't a real world scenario we really need to address - anyone
> doing such sequential IO should be using fwrite(3), not write(2).
> i.e. fwrite(3) buffers the writes in userspace to minimise the
> number of write(2) syscalls, and the problem goes away.
> 
> However, there is a small change we can make to improve the
> situation - removing the expensive lock operation on the change
> counter update.  All inode version counter changes in XFS occur
> under the ip->i_ilock during a transaction, and therefore we
> don't actually need the spin lock that provides exclusive access to
> it through inc_inode_iversion().
> 
> Hence avoid the lock and just open code the increment ourselves when
> logging the inode.

Well, ok.  Maybe worth a note about why the unlocked read is 99.9999% ok...

Reviewed-by: Eric Sandeen <sandeen@redhat.com>

> Reported-by: Michael L. Semon <mlsemon35@gmail.com>
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> ---
>  fs/xfs/xfs_trans_inode.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/xfs/xfs_trans_inode.c b/fs/xfs/xfs_trans_inode.c
> index 1bba7f6..50c3f56 100644
> --- a/fs/xfs/xfs_trans_inode.c
> +++ b/fs/xfs/xfs_trans_inode.c
> @@ -111,12 +111,14 @@ xfs_trans_log_inode(
>  
>  	/*
>  	 * First time we log the inode in a transaction, bump the inode change
> -	 * counter if it is configured for this to occur.
> +	 * counter if it is configured for this to occur. We don't use
> +	 * inode_inc_version() because there is no need for extra locking around
> +	 * i_version as we already hold the inode locked exclusively for
> +	 * metadata modification.
>  	 */
>  	if (!(ip->i_itemp->ili_item.li_desc->lid_flags & XFS_LID_DIRTY) &&
>  	    IS_I_VERSION(VFS_I(ip))) {
> -		inode_inc_iversion(VFS_I(ip));
> -		ip->i_d.di_changecount = VFS_I(ip)->i_version;
> +		ip->i_d.di_changecount = ++VFS_I(ip)->i_version;
>  		flags |= XFS_ILOG_CORE;
>  	}
>  
> 

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